As I work through the details of my "Bolt-On" gear swap idea, I have become something of an expert in AF05E and AF16E transmission parts trivia.
According to the OEM Parts catalog I've studied:
http://www.mrcycles.com/fiche_section_detail.asp
It seems the tranny cover from 1985 until 1997 carried the same basic part #:
21301-GN2-0*0 where series 1, 2, and 3 replace the *.
Then in 1998 the part is evidently interchangeable with
21301-GBL-870
Which incidentally lists for $12 cheaper than the item it supercedes. Both are listed under that model year. From there on the *-GBL-870 appears to be the part listed for all later SA50 models. In the 2001 catalog it was $2. cheaper still. By now it must be free!
This may explain some issues with shaft length described in gear swap posts. It appears that simply dropping the shim washers solves the problem in many cases - Pun intended...
Anyway, YMMV